Data collection with mobile elements in wireless sensor networks brings in new opportunities to reduce and balance the energy consumption of sensor nodes, however, it may result in high data collection latency. The optimal scheduling of mobile element's limited mobility is of great importance to reduce this latency, and a lot of research efforts have been made on it. Focusing on the on-demand data collection scenario, where data collection requests appear progressively, and based on the fact that several nearby requests can be combined and served from the same collection site, we propose a simplified combine-skip-substitute (CSS) scheme, which is shown to be able to reduce the data collection latency greatly. We also analyze the probability for the combination to happen, and how many requests can be combined, to gain more insights in its performance. The efficacy of the proposed scheme and the accuracy of the analytical results are verified through extensive simulation.